In low setting you can't hear it *bonus* Lightweight & easy to relocate. It has made a huge difference with my allergies. 
Two things that I don't care for are: the top control needs to be quite firm (this is a complaint of many users. Hope they change this; as I need another unit. I just leave it on the lowest setting & unplug it when changing rooms.  When plugged back in it is on the same setting, so I don't have to deal with this problem). The other issue is the very bright very blue light is not appropriate for use in bedrooms; unless you need a night light. Several layers of painters tape are now covering it ~deffinatly detracting from the sleek design.